Method for mems device fabrication and device formed

ABSTRACT

The present invention generally relates to methods for producing MEMS or NEMS devices and the devices themselves. A thin layer of a material having a lower recombination coefficient as compared to the cantilever structure may be deposited over the cantilever structure, the RF electrode and the pull-off electrode. The thin layer permits the etching gas introduced to the cavity to decrease the overall etchant recombination rate within the cavity and thus, increase the etching rate of the sacrificial material within the cavity. The etchant itself may be introduced through an opening in the encapsulating layer that is linearly aligned with the anchor portion of the cantilever structure so that the topmost layer of sacrificial material is etched first. Thereafter, sealing material may seal the cavity and extend into the cavity all the way to the anchor portion to provide additional strength to the anchor portion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional patent application of U.S. patentapplication No. 13/349,696 (CK071) filed Jan. 13, 2012, whichapplication claims benefit of U.S. Provisional Patent Application Ser.No. 61/432,628 (CK071L), filed Jan. 14, 2011, which is hereinincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention generally relate to a method ofrelease etching a micro-electromechanical system (MEMS) ornano-electromechanical system (NEMS) contained in a cavity.

2. Description of the Related Art

The key part of any MEMS or NEMS fabrication process is the release ofthe structure from the surrounding sacrificial material to therebyenable motion of the structure within the cavity. The step of thesacrificial material removal is also called sacrificial etching orrelease etching. The sacrificial etching or releasing etching iscritical from a technological and economical point of view.

On the technical side, the device may easily break during thesacrificial etching or release etching. On the economical side, therelease etch step is often the longest step in the manufacturing processwith release etching time lasting up to a few hours per wafer. The longetching time results in a low throughput rate and is very likely to hurtthe cost per unit if the cost of ownership of the release equipment ishigh or if additional capital expenditures are required.

Therefore, there is a need in the art for a method of release etching aMEMS or NEMS contained within a cavity and for a stable devicemanufactured using such a method.

SUMMARY OF THE INVENTION

The present invention generally relates to methods for producing metalMEMS or NEMS devices and the devices themselves. A thin layer of amaterial having a lower recombination coefficient as compared to thecantilever structure may be deposited over the cantilever structure, theRF electrode and the pull-off electrode. The thin layer decreases theoverall etchant recombination rate of the etchant gas within the cavity,and thus increases the etching rate of the sacrificial material withinthe cavity. The etchant itself may be introduced through an opening inthe encapsulating layer that is linearly aligned with the anchor portionof the cantilever structure so that the topmost layer of sacrificialmaterial completes the etching process first. Thereafter, sealingmaterial may seal the cavity and extend into the cavity all the way tothe anchor portion to provide additional strength to the anchor portion.

In one embodiment, a device includes a substrate having one or more viasextending therethrough that are filled with an electrically conductivematerial that is electrically connected to one or more layers below thesubstrate. The device also includes a wall coupled to the substrate andextending in a direction perpendicular to a top surface of the substrateand a roof coupled to the wall such that the wall, roof and substratecollectively enclose a cavity. The device additionally includes acantilever structure enclosed within the cavity, movable within thecavity, and coupled to the electrically conductive material. Thecantilever structure comprises a multilayer structure having a firstlayer of a material that has a first recombination coefficient and asecond layer that faces the roof. The second layer comprises a materialthat has a second recombination coefficient that is lower than the firstrecombination coefficient.

In another embodiment, a method includes depositing a first insulatinglayer over a substrate, etching the first insulating layer to expose atleast a portion of the substrate and form a via, depositing a firstsacrificial layer over the first insulating layer, and etching the firstsacrificial layer to at least partially define outer boundaries for acavity to be formed and to expose the substrate through the via. Themethod also includes depositing a structural layer on the substrate andon the first sacrificial layer. The structural layer comprises amaterial having a first recombination coefficient. The method alsoincludes etching the structural layer to at least partially define theshape of a cantilever device and expose at least a portion of the firstsacrificial layer and depositing a second insulating layer on thestructural layer. The second insulating layer comprises a materialhaving a second recombination coefficient that is lower than the firstrecombination coefficient. The method also includes etching the secondinsulating layer and the structural layer to at least partially definethe shape of the cantilever device and to expose at least a portion ofthe first sacrificial layer, depositing a second sacrificial layer onthe first sacrificial layer and on the second insulating layer, etchingthe second sacrificial layer such that the first sacrificial layer andthe second sacrificial layer collectively define the shape of the cavityto be formed, and depositing a third insulating layer over the secondsacrificial layer. The method additionally includes depositing anelectrode layer on the third insulating layer, depositing anencapsulating layer on the electrode layer, etching an opening in theencapsulating layer, introducing an etchant through the opening, andetching the first sacrificial layer and the second sacrificial layer toremove the first sacrificial layer and the second sacrificial layer, toform the cavity and free the cantilever device to move within thecavity.

In another embodiment, a method includes depositing a first sacrificiallayer over a substrate, removing material from the first sacrificiallayer to form a via and expose at least a portion of the substrate,forming a cantilever structure over the first sacrificial layer andwithin the via, depositing a second sacrificial layer over thecantilever structure, and depositing an encapsulating layer over thesecond sacrificial layer, first sacrificial layer and the substrate toform at least a portion of a cavity boundary. The method additionallyincludes removing at least a portion of the encapsulating layer toexpose the second sacrificial layer and form an opening that is axiallyaligned with the via, introducing an etching gas to the secondsacrificial layer through the opening, and etching the secondsacrificial layer and the first sacrificial layer to form a cavity andfree the cantilever structure within the cavity.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIGS. 1A-1I are schematic cross-sectional views of a MEMS device atseveral stages of processing.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements disclosed in oneembodiment may be beneficially utilized on other embodiments withoutspecific recitation.

DETAILED DESCRIPTION

The present invention generally relates to methods for producing MEMS orNEMS devices and the devices themselves. A thin layer of a materialhaving a lower recombination coefficient as compared to the cantileverstructure may be deposited over the cantilever structure, the RFelectrode and the pull-off electrode. The thin layer decreases theoverall etchant recombination rate of the etchant gas within the cavity,and thus increases the etching rate of the sacrificial material withinthe cavity. The etchant itself may be introduced through an opening inthe encapsulating layer that is linearly aligned with the anchor portionof the cantilever structure so that the topmost layer of sacrificialmaterial completes the etching process first. Thereafter, sealingmaterial may seal the cavity and extend into the cavity all the way tothe anchor portion to provide additional strength to the anchor portion.

One overall problem with cantilever devices containing metal portionsenclosed within cavities is that the cavity as well as the encloseddevice has a large fractional area made of metal. If the sacrificialmaterial is an organic material and the release chemistry used ishydrogen and/or oxygen based, the etching ions may recombine due tointeraction with the metal surfaces and thus lower the etching rate. Thedesign here involves spacing apart all uncoated metal or metal nitridesurfaces as much as possible within design/device constraints.

FIGS. 1A-1I are schematic cross-sectional views of a MEMS device atseveral stages of processing. The device includes a substrate 102. Thesubstrate 102 is shown as a generic substrate, but it is to beunderstood that the substrate may be a single layer material such as asemiconductor wafer or a multiple layer structure such as acomplementary metal oxide semiconductor (CMOS) device. For a singlelayer material, the device may be built and then attached to a devicesuch as a CMOS device.

Embedded within the substrate 102 are several structures. There areelectrical connections/contacts 104, 106 that provide electricalconnection from the device to layers below the substrate 102. There arealso pull-in electrodes 108, 110 that are used to pull the device from aposition spaced a first distance from the RF electrode 112 to a seconddistance closer to the RF electrode 112. The electrical connections 104,106, pull-in electrodes 108, 110 and the RF electrode 112 may all beformed by removing material from the substrate 102 by a process such asetching, blanket depositing an electrically conductive material, andremoving excess electrically conductive material by a process such asetching or chemical mechanical polishing. Suitable electricallyconductive material that may be used include copper, aluminum, titanium,tungsten, titanium nitride, titanium aluminum nitride, and combinationsthereof. The substrate material that the electrically conductivematerial is formed within may include electrically insulating materialsuch as silicon oxide, silicon nitride, silicon oxynitride, andcombinations thereof. Alternatively, the electrical connections 104,106, pull-in electrodes 108, 110 and the RF electrode 112 may all beformed by first blanket depositing an electrically conductive materialover the substrate 102. Thereafter, excess portions of the electricallyconductive material are removed by a process such as etching to form thefinal shape of the electrical connections 104, 106, pull-in electrodes108, 110 and the RF electrode 112. Thereafter, an electricallyinsulating material may be deposited over the exposed substrate 102 andelectrical connections 104, 106, pull-in electrodes 108, 110 and the RFelectrode 112 by a blanket deposition process. Excess electricallyinsulating material may then be removed by a process such as etching orCMP.

Design constraints or material-related MEMS characteristics make itdifficult to change either the cavity or the MEMS material. However,recombination being a surface phenomena, one may work to reduce therecombination coefficient of some encountered surfaces. This could beachieved by coating some critical surfaces with a material having a lowrecombination coefficient for the etchants involved. Consideringhydrogen or oxygen radicals as etchants, an effective recombinationinhibitor will typically be an insulating material or a material withsaturated lattice bonds. These include, but are not limited to, siliconnitride, silicon oxide, silicon oxynitride as well as some insulatingmetal oxide (aluminum oxide, titanium oxide, hafnium oxide) orinsulating metal nitride (aluminum nitride).

The electrically conductive material may have a different recombinationcoefficient than the electrical insulating material. As such, during anetching process to remove sacrificial material from the cavity, theetching radicals may recombine when exposed to the electricallyconductive material. Because the radicals recombine, the etching ratedecreases and throughput suffers. Therefore, an electrically insulatinglayer 114 is deposited over the substrate 102 as well as the electricalconnections 104, 106, the pull-in electrodes 108, 110 and the RFelectrode 112. The electrically insulating layer 114 has a thickness ofbetween about 100 nm and about 225 nm. In one embodiment, the thicknessmay be between about 175 nm and about 210 nm. The insulating layer 114needs to be thin enough to permit the pull-in electrodes 108, 110 aswell as the RF electrode 112 to function properly. The thickness ofinsulating layer 114 and the material chosen for the insulating layer114 is driven by the capacitance requirements of the device. Suitableelectrically insulating material that may be utilized include siliconnitride, silicon oxide or combinations thereof. It is contemplated thatother electrically insulating material may be utilized as well so longas the electrically insulating material has a much slower etch rate thanthe sacrificial material that will be formed and removed thereover.

Over the electrically insulating layer 114, a sacrificial layer 116 maybe deposited. The sacrificial layer 116 is a layer that is used to helpdefine not only the shape of the cantilever device, but also the shapeof the cavity in which the device is formed. Suitable materials that maybe used for the sacrificial layer 116 include silicon containingcompounds such as amorphous silicon, silicon dioxide, silicon nitride,silicon oxide, silicon oxynitride, spin-on glass or spin on dielectriccontaining a long chain molecule with a carbon backbone, andcombinations thereof. It is important that the material chosen for theelectrically insulating layer 114 and the material selected for thesacrificial layer 116 have different etch rates so that the sacrificiallayer 116 will be removed without removing too much material from theelectrically insulating layer 114. The sacrificial layer 116 may bedeposited by methods including atomic layer deposition (ALD), chemicalvapor deposition (CVD), physical vapor deposition (PVD), spin coatingand other well known deposition methods. After deposition, thesacrificial layer 116 may then be exposed to a high temperature toanneal or cure the sacrificial layer 116.

Vias 118 are then formed through the sacrificial layer 116 and theelectrically insulating layer 114 to expose at least a portion of theelectrical connections 104, 106. The vias 118 may be formed by placingor forming a mask over the sacrificial layer 116 and then etchingthrough the mask openings to remove material. It is to be understoodthat the insulating layer 114 may be etched prior to deposition of thesacrificial layer 116 if desired by using a technique as discussedabove. The resulting structure is shown in FIG. 1A.

Following formation of the vias 118, material for a first structurallayer 120 of the cantilever device is deposited. The material isdeposited into the vias 118 onto the exposed electrical contacts 104,106 to provide an electrical connection to the device. The material,after deposition, is shaped to form the desired final structure for thefirst structural layer. The material is shaped by depositing or forminga mask thereover and then etching exposed material. The mask is thenremoved leaving the first structural layer 120 as shown in FIG. 1B.Suitable materials that may be used for the first structural layerinclude titanium nitride, titanium aluminum, tungsten, copper, titaniumaluminum nitride aluminum and combinations thereof and multilayerstructures such as titanium nitride-aluminum-titanium nitride. Thematerial for the first structural layer 120 may be deposited by asuitable deposition process such as PVD.

Because sacrificial material that is formed over the first structurallayer 120 will be removed, the first structural layer 120 may be exposedto etchant during the removal process. The material for the firststructural layer 120 may permit recombination of the etching radicalsand thus, reduce the etching rate and decrease throughput. Similar tothe insulating layer 114 discussed above, a second insulating layer 122may be deposited over the first structural layer 120 such that thesecond insulating layer 122 has a lower recombination coefficient ascompared to the first structural layer 120. The depositing methods andmaterials that are suitable for insulating layer 114 are suitable forthe second insulating layer 122 as well, although the materials andmaterial thicknesses need not be identical. The second insulating layer122 is then etched to remove material that is not covering the firststructural layer 120 and to exposes portions of the first sacrificiallayer 116. Vias may be etched through the second insulating layer 122 atthis time if desired or may be etched following formation of the secondsacrificial layer 124.

The second sacrificial layer 124 may then be formed over both the secondinsulating layer 122 and the exposed first sacrificial layer 116. Thesecond sacrificial layer 124 may comprise materials as discussed abovewith regards to the first sacrificial layer 116 and be formed by similarmethods. However, it is to be understood that the methods and materialfor layers 116, 124 need not be identical. The second sacrificial layer124 is etched to define at least a portion of the boundary for thecavity and to at least partially define the shape of the next structurallayer to be formed. Vias 126 may be etched through the secondsacrificial layer 124 (as well as the second insulating layer if notalready formed) to expose the first structural layer 120 as shown inFIG. 1C. The vias 126 will eventually be filled with material that willsupport the next structural layer and provide electrical connectionbetween the structural layers.

Within the vias 126 and over the second sacrificial layer 124, materialforming the second structural layer 128 is deposited. The material isshaped to form the desired final structure of the second structurallayer 128 as shown in FIG. 1D. The material within the vias 126 formsposts to space the second structural layer 128 from the first structurallayer 120 and create a waffle-like structure. Suitable depositionmethods and materials that may be used include those discussed abovewith regards to first structural layer 120. It is to be understood thatthe material and deposition methods used for the first structural layer120 and the second structural layer 128 need not be identical. Becausesacrificial material that is formed over the second structural layer 128will be removed, the second structural layer 128 may be exposed toetchant during the removal process. The material for the secondstructural layer 128 may permit recombination of the etching radicalsand thus, reduce the etching rate and decrease throughput. Similar tothe insulating layer 114 discussed above, a third insulating layer 129may be deposited over the second structural layer 128 such that thethird insulating layer 129 has a lower recombination coefficient ascompared to the second structural layer 128.

Similar to both the first and second insulating layers 114, 122, thethird insulating layer 129 has a lower recombination coefficient ascompared to the second structural layer 128. Suitable materials anddeposition methods that may be used include the methods and materialsdiscussed above with regards to the first and second insulating layers114, 122. It is to be understood that the methods and materials for theinsulating layers need not be identical. The third insulating layer 129and the second structural layer 128 are etched to form the finalcantilever shape as shown in FIG. 1E and expose the second sacrificiallayer 124.

A third sacrificial layer 130 is then formed over the exposed secondsacrificial layer 124 and the third insulating layer 129. Suitabledeposition methods and materials as are described above for the firstand second sacrificial layers 116, 124 may be used, although, themethods and materials need not be identical. The final shape of thefirst sacrificial layer 116, the second sacrificial layer 124 and thethird sacrificial layer 130 collectively define the shape of the cavityto be formed.

A fourth insulating layer 132 is then deposited over the thirdsacrificial layer 130. The fourth insulating layer 132 is used toprotect the pull-off electrode from exposure to the etching ions thatare used to remove the sacrificial material. Thus, the fourth insulatinglayer 132 may be fabricated by a process and of a material as describedabove with regards to the first, second and third insulating layers 114,122, 129. However, it is to be understood that the methods and materialsfor the insulating layers need not be identical.

An electrically conductive layer is then deposited over the fourthinsulating layer 132 to form the pull-off electrode 134. While notshown, the pull-off electrode 134 may be electrically connected tolayers either below the substrate 102 or above the pull-off electrode134. Vias 136 may then be etched through both the pull-off electrode 134as well as the fourth insulating layer 132. The vias 136 are formed sothat they are linearly aligned with the electrical contacts 104 as shownin FIG. 1E. As will be discussed below, having the vias 136 linearlyaligned with the electrical contacts 104, 106 assist in increasing thestructural integrity of the device.

An etching gas may then be introduced through the vias 136. In order toget the highest possible etch rate for a given release hole, the releasechemistry may be optimized. Adding oxygen to the etching gas was seen tohave a valuable impact on the release etch rate. It has been found thatutilizing a higher pressure and up to 10 percent diatomic oxygen in theetching gas could improve etching rates for carbon based sacrificialmaterial such as PolyArylene Ether (PAE) based sacrificial materials.This could be explained by multiple reasons such as faster etch rate ata given partial pressure (lower activation energy of the limited step),etchant yield increase through positive catalyzing action with thehydrogen plasma, or partial passivation of the metal surface limitingthe recombination rate of hydrogen and/or oxygen on metal surfaces.

During the etching process, the etchant consists of molecular hydrogenor oxygen. These species are generated at low to medium pressure by aplasma reactor. At the pressure involved (i.e., about 1 mTorr to about50 mTorr), the mean free path is large when compared to any criticaldimension of the cavity. This means that atom-atom collisions in the gasphase (H+H−>H2) are a rare event and does not lead to a depletion ofatomic species. On the other hand, atom-surface collisions are extremelyfrequent and the outcome off the collision is determined by thesurface's nature. Consider, for example, an etchant E (E being hydrogen(H) or oxygen (O)) and a surface S. Atom-surface collisions will lead tothe adsorption of the etchant on the surface: E+S−>E_(ads). On thesacrificial layer surface, adsorbed etchants react on the organicmaterial and etch it (C_(x)N_(y)H_(z)O(s)+H_(ads) and/or O_(ads)−>CH₄(g), NO(g), NO₂(g), CO₂(g), H₂O(g), etc.)

On a metal surface, adsorbed etchants potentially recombine(E_(ads)−>E₂+2S). The efficiency at which recombination occurs onsurfaces can be characterized by the recombination coefficient. Thehigher the recombination coefficient, the greater is the loss of atomicetchants. This ultimately leads to a reduction of the flux of availableetchants and hence, a decrease in sacrificial etch rate. Metals on whichhydrogen and/or oxygen recombine include, but are not limited to,copper, platinum, titanium, titanium nitride, alloys of titaniumaluminum (Ti_(x)Al_(y)), alloys of titanium aluminum nitride(Ti_(x)Al_(y)N), tantalum nitride, chromium, and tungsten.

Etching the cavity is one of the longest processing steps and is limitedby the loss of active etch ions at the metal surfaces. The insulatinglayer over the metal surface solves the problem. The etch holes at thetop permit removal of the top sacrificial layer first because byremoving the top sacrificial layer before the bottom sacrificial layer,the device is not stuck to the roof during the release. The roof of thecavity may bend up during the release because the release is a hotprocess. If the MEMS device were release bottom first with the top stillattached to the roof, then the movement of the cavity could destroy thevias holding the MEMS device to the electrical contacts below. Byreleasing the top first, the problem is solved. The etch holes arepositioned so that they are above the MEMS vias underneath. Then, whenfiled with material, the filling adds to the strength of the MEMS deviceheld at each end.

Because of the location of the vias 136, the third sacrificial layer 130will be removed first to begin to form the cavity as shown in FIG. 1F.Removing the third sacrificial layer 130 first permits the thirdinsulating layer 129 to be spaced from the fourth insulating layer 132and not directly connected. During the etching, the temperature and eventhe pressure within the cavity 138 may increase and cause an expansionof the fourth insulating layer 132 and pull-off electrode 134. If thethird sacrificial layer 130 is removed last, then the device may bedamaged during any expansion.

Thereafter, the second sacrificial layer 124 is removed as shown in FIG.1G and the first sacrificial layer 116 is removed to completely form thecavity 138 and free the device as shown in FIG. 1H. A sealing material140 may then be deposited to seal the vias 136 and even fill the vias136 such that the sealing material is deposited on the exposed secondinsulating layer 122. The sealing material 140 thus not only seals thecavity 138 but also provides additional structural support to thedevice. The sealing material 140 that extends through the vias 136provides additional support of the roof of the cavity 138 and helpsmaintain the structural integrity of the device by preventing the devicefrom breaking apart from the electrical contacts 104 when the devicemoves within the cavity in response to an electrical bias applied toeither the pull-in electrodes 108, 110 or the pull-off electrode 134.When a bias is applied to either the pull-in electrodes 108, 110 or thepull-off electrode, the device moves either closer to or further awayfrom the RF electrode 112. A great deal of stress may be placed on thedevice at the connection to the electrical contacts 104, 106. Thesealing material 140, because it is connected to the second insulatinglayer 122, reduces the stress by shifting the location where thegreatest stress occurs to a location spaced from the electrical contacts104, 106.

It is to be understood that while the thin layer of material that has alower recombination coefficient as compared to the cantilever structurehas been discussed as being deposited over the cantilever structure, itis contemplated that the thin layer could be deposited such that thethin layer is also under the cantilever structure or even encapsulatingthe cantilever structure. It is also contemplated to add release holesat locations corresponding to each sacrificial layer to increase theetching rate. The advantages of utilizing the top release holes and bydepositing the insulating material over the exposed metal surfacesinclude faster etching that translates into lower over etch gradientthrough the free-standing element and higher throughput and lowermanufacturing cost.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. A device, comprising: a substrate having one or more vias extendingtherethrough that are filled with an electrically conductive materialthat is electrically connected to one or more layers below thesubstrate; a wall coupled to the substrate and extending in a directionperpendicular to a top surface of the substrate; a roof coupled to thewall such that the wall, roof and substrate collectively enclose acavity; a cantilever structure enclosed within the cavity, movablewithin the cavity, and coupled to the electrically conductive material,the cantilever structure comprising a multilayer structure having afirst layer of a material that has a first recombination coefficient anda second layer that faces the roof, the second layer comprising amaterial that has a second recombination coefficient that is lower thanthe first recombination coefficient.
 2. The device of claim 1, whereinthe first layer is selected from the group consisting of titaniumaluminum nitride, titanium nitride and combinations thereof.
 3. Thedevice of claim 2, wherein the second layer is selected from the groupconsisting of silicon nitride, silicon oxide and combinations thereof.4. The device of claim 1, further comprising a filling material coupledto the cantilever structure at one or more locations that are directlyabove the one or more vias, the filling material extending from thecantilever structure to the roof.
 5. The device of claim 1, furthercomprising: an electrode coupled to the roof; and an electricallyinsulating layer coupled to the electrode such that the electrode isisolated from the cavity by the electrically insulating layer.
 6. Thedevice of claim 1, wherein the substrate has one or more electrodesdisposed therein, the device further comprising an electricallyinsulating layer disposed on the substrate between the cantileverstructure and the one or more electrodes.
 7. The device of claim 1,wherein the cantilever structure comprises one or more anchor portionscoupled to the electrically conductive material, a waffle portion, and abending portion coupled between the one or more anchor portions and thewaffle portion.
 8. The device of claim 1, wherein the cantileverstructure comprises: a first structural layer; a first electricallyinsulating layer disposed on the first structural layer; a plurality ofposts extending from the first electrically insulating layer; the firstlayer of a material that has a first recombination coefficient disposedon the plurality of posts; and the second layer.